Polymer filtration control agents can not only effectively control the filtration loss of drilling fluids but also guarantee the performance of the drilling fluids with a normal density (<1.5 g/cm3 in general) and play an important role in drilling operation. However, when they are used in a high density drilling fluid (>1.8 g/cm3 in general) or an ultra high density drilling fluid (>2.5 g/cm3 in general), usually the drilling fluid is thickened and its rheological behavior is worsened due to a high molecular weight and a large viscosity effect.
In an ultra high density drilling fluid, the weight fraction of its weighting material accounts for about 70% of the total weight of the drilling fluid (calculated by taking the drilling fluid with a density of 2.5 g/cm3 prepared from barite with a density of 4.2 g/cm3 for example), while the free water in the system is relatively small, so improper use of the additives that maintain good performance of the drilling fluid may cause rise of viscosity and yield point of the drilling fluid, worsen of the quality of filter cakes and difficulty in controlling filtration loss at high temperature and high pressure. The ultra-low viscosity carboxymethyl cellulose (ultra-low viscosity CMC) and ultra-low viscosity polyanionic cellulose (ultra-low viscosity PAC) filtration control agent currently applied on site may cause quick rise of the viscosity and yield point of an ultra high density drilling fluid and worsen of the rheological behavior of the drilling fluid even if the amount is small. With regard to filtration control agent sulfonated phenolic resin (SMP), viscosity reducer sulfonated lignite (SMC) and sulfonated tannin extract (SMK) as well as their modified products often used on site, when solid content and mineralization are high, usually they must be used in a combined way and the dosage of filtration control agent and/or viscosity reducer must be increased so that they can show an effect. As a result, the cost of the drilling fluid is increased, the rheological behavior is worsened and labor intensity is increased. More importantly, its filtration loss reduction effect is not obvious under the condition of high salt content and high temperature.
Lignite features rich resources and low price. The products made by using it as a basic raw material are often used in drilling fluids. For example, literature “research of the synthesis and performance of heat and salt-resistant filtration control agent SHK-AN” discloses a method for preparing filtration control agent SHK-AN which shows a good effect in temperature and salt resistance and filtration loss reduction, but its synthetic process is complex and the lignite powder needs to undergo oxidative degradation by nitric acid, extraction by alkali and sulfomethylation, the obtained sulfomethyl potassium humate needs to undergo graft polymerization with acrylonitrile obtained after hydrolysis of waste acrylic fiber, and then concentration, drying and pulverization are needed in order to obtain SHK-AN graft copolymer. The research indicates this graft copolymer has a high molecular weight and a large viscosity effect.
Literature “synthesis and evaluation of filtration control agent AMPS/AM/sodium humate” discloses a method for preparing filtration control agent AMPS/AM/sodium humate and specifically discloses that sodium humate is prepared from lignite and the obtained sodium humate is copolymerized with 2-acrylamide-2-sodium methyl-propanesulfonate and acrylamide to obtain filtration control agent AMPS/AM/sodium humate. The filtration control agent has a desirable ability in reducing filtration loss in fresh water, but its salt resistance is poor. Literature “research and application of heat and salt resistant filtration control agent SHK” discloses a method for preparing filtration control agent SHK from lignite and amide derivative as well as complexing agent, cross-linking agent, formaldehyde, soda ash and pyrosulfurous acid. The filtration control agent has high resistance to temperature and salt, strong thinning capacity and good wall building and lubricating properties. However, the maximum density of the drilling fluid using it is 1.97 g/cm3.